The Foundations of Chemistry

130 CHAPTER 4: Some Types of Chemical Reactions

Dissociationrefers to the process in which a solid ionic compound,such as NaCl, sepa-

rates into its ions in solution:

H 2 O

NaCl(s)8888nNa
(aq)
Cl(aq)

Molecular compounds, for example pureHCl, exist as discrete molecules and do not

contain ions; however, many such compounds form ions in solution. Ionizationrefers to

the process in which a molecular compoundseparates or reacts with water to form ions in

solution:

H 2 O

HCl(g)8888nH
(aq)
Cl(aq)

Three major classes of solutes are strong electrolytes: (1) strong acids, (2) strong

bases, and (3) most soluble salts. These compounds are completely or nearly completely

ionized (or dissociated) in dilute aqueous solutions,and therefore are strong electrolytes.

An acidcan be defined as a substance that produces hydrogen ions, H
, in aqueous

solutions. We usually write the formulas of inorganic acids with hydrogen written first.

Organic acids can often be recognized by the presence of the COOH group in the formula.

A baseis a substance that produces hydroxide ions, OH, in aqueous solutions. A saltis

a compound that contains a cation other than H
and an anion other than hydroxide ion,

OH, or oxide ion, O^2 (see Table 2-3 on page 55). As we will see later in this chapter,

salts are formed when acids react with bases.

2 Strong and Weak Acids

As a matter of convenience we place acids into two classes: strong acids and weak acids.

Strong acidsionize (separate into hydrogen ions and stable anions) completely, or very

nearly completely, in dilute aqueous solution. The seven common strong acids and their

anions are listed in Table 4-5. Please learn this short list; you can then assume that other

acids you encounter are weak.

TABLE 4-5 Common Strong Acids and Their Anions

Common Strong Acids Anions of These Strong Acids

Formula Name Formula Name

HCl hydrochloric acid Cl chloride ion

HBr hydrobromic acid Br bromide ion

HI hydroiodic acid I iodide ion

HNO 3 nitric acid NO 3  nitrate ion

HClO 4 perchloric acid ClO 4  perchlorate ion

HClO 3 chloric acid ClO 3  chlorate ion

H HSO^4  hydrogen sulfate ion

2 SO 4 sulfuric acid SO 42  sulfate ion

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Recall that ionsare charged particles.
The movement of charged particles
conducts electricity.

In Chapter 10, we will see that it is
appropriate to represent H
(aq) as
H 3 O
to emphasize its interaction
with water.

Acids and bases are further identified
in Subsections 2, 3, and 4. They are
discussed in more detail in Chapter 10.

Positively charged ions are called
cations,and negatively charged ions are
called anions(Section 2-3). The
formula for a salt may include H or
OH, but it mustcontain another cation
andanother anion. For example,
NaHSO 4 and Al(OH) 2 Cl are salts.

Many properties of aqueous solutions
of acids are due to H
(aq) ions. These
are described in Section 10-4.